Core Java 面试题与答案

问题 66. How do you know if an explicit object casting is needed?

If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:

Object a; Customer b; b = (Customer) a;

问题 67. What are native methods? How do you use them?

Native methods are methods written in other languages like C, C++, or even assembly language. You can call native methods from Java using JNI. Native methods are used when the implementation of a particular method is present in language other than Java say C, C++. To use the native methods in java we use the keyword native
public native method_a(). This native keyword is signal to the java compiler that the implementation of this method is in a language other than java. Native methods are used when we realize that it would take up a lot of rework to write that piece of already existing code in other language to Java.

问题 68. What is the use of transient?

It is an indicator to the JVM that those variables should not be persisted. It is the users responsibility to initialize the value when read back from the storage.

问题 69. What is Externalizable?

Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

问题 70. What modifiers may be used with an inner class that is a member of an outer class?

A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.